Six months later....I finally finished the 2" spacer back. With the spacer back, no vignetting on 7x11 with a 450mm or 24 inch (610mm) lens (haven't tried the 30 inch yet). Without the spacer back, no vignetting on 7x11 with 300mm and shorter lenses.
I made the spacer back out of 2" x 1/2" balsa wood and covered it with 1/16" inch basswood. Weighs 1/2 pound.
Two bonuses--with the spacer back I can: (1) reverse the 7x11 back to portrait orientation, and (2) I gain 2" of extra "bellows;" the latter will come in handy when using the 30" lens even with 8x10 or the 4x5 reducing back (I ran out of bellows a few months back trying to photograph petroglyphs with the 30" lens and 4x5 reducing back across a roped off area--not enough bellows to focus the 30" lens at 20'.).